1. Field of the Invention
The present invention relates to an alignment process of an alignment film for a liquid crystal display device, and more particularly, to an alignment method in which an ion beam is irradiated onto an alignment film using a bar-type ion beam gun.
2. Discussion of the Related Art
As new and modern image displays such as high definition televisions (TV) are developed, researches on flat panel displays, such as liquid crystal display devices (LCDs), electro luminescence displays (ELDs), vacuum fluorescence displays (VFDs), plasma display panels (PDPs) and the like, are actively being conducted.
LCDs are a representative display of these flat panel displays, which have such features as slim profile, low price, low power consumption, etc., and extend applications to vehicle monitors and color TVs as well as laptop computers and pocket computers.
An LCD has a structure in which an upper color filter substrate and a lower thin film transistor (TFT) substrate are disposed facing each other and a liquid crystal layer having a dielectric anisotropy is interposed between the upper color filter substrate and the lower TFT substrate. The LCD operates by switching thin film transistors (TFTs) electrically connected to several hundred thousand pixels to apply voltages to corresponding pixels through pixel selection address lines.
A method of fabricating the LCD includes a transistor process, a color filter process, a liquid crystal (LC) cell process and a module process.
The transistor process is used for forming a TFT array on a glass substrate by repeatedly performing deposition, photolithography and etching operations. The color filter process is used for forming a color filter layer including red (R), green (G) and blue (B) color filters on a glass substrate including a black matrix layer thereon using dye or pigment, and for forming an ITO film serving as a common electrode.
The LC cell process is used for attaching a glass substrate where the transistor process is completed and a glass substrate where the color filter process is completed such that a space is formed between the two glass substrates, and injecting liquid crystal into the space to form an LC cell. The module process is used for forming a circuit part for signal processing, for connecting the circuit part with a TFT-LCD panel using a mounting technique, and for assembling the TFT-LCD panel and the circuit part with structural elements to fabricate an LCD module.
In more detail, the LC cell process includes aligning an alignment film, attaching the two substrates, and injecting liquid crystal. The alignment step includes cleaning the substrates, coating an alignment film, hardening the alignment film, rubbing the alignment film, cleaning the alignment film and the like.
The alignment step will now be described with reference to the accompanying drawings. FIG. 1 is a flow diagram illustrating an alignment method according to the related art, and FIG. 2 is a cross-sectional view illustrating a rubbing method according to the related art.
Referring to FIG. 1, a substrate cleaning step S100 is first performed to remove foreign particles from an upper substrate or a lower substrate. The substrate cleaning step S100 is performed by spraying deionized water onto an upper surface of the substrate through a nozzle to remove water-soluble impurities from the upper surface of the substrate, or is performed using a cleaning apparatus provided with a UV processing part to remove organic impurities.
Next, in an alignment film printing step S101, a raw material for an alignment film is printed on an effective display area of the upper surface of the substrate to a predetermined thickness by a spin-coating method or a roll-coating method.
Next, in a pre-baking step S102, heat is applied to the raw material to volatize the solvent contained in the raw material. When the solvent of the raw material is volatized, a pretilt angle appears on an upper surface of the printed raw material and dust is prevented from being adhered on the printed raw material during the subsequent steps.
An alignment film hardening step S103 is subsequently performed, in which the printed raw material is heated in a temperature range of 80 to 200° C., which is higher than a temperature range employed in the pre-baking step S102, so that the printed raw material is hardened.
Next, an alignment film rubbing step S104 is performed using a rubbing roll 30 having grooves engraved at a constant interval, as illustrated in FIG. 2. In the alignment film rubbing step S104, the upper surface of the alignment film printed on a substrate 15 is rubbed in a selected direction to form grooves. The rubbing step S104 is performed by moving the substrate 15 loaded on a transfer plate 31 or the rubbing roll 30 in a selected direction.
Lastly, an alignment film cleaning step S105 is performed so as to remove foreign particles that may exist on the substrate having the alignment film thereon. With the alignment film cleaning step S105, the alignment process is completed.
As described above, the rubbing method illustrated in FIG. 2 is generally employed to align an alignment film according to the related art TFT-LCD. In the rubbing method, an alignment film 10 is coated on the substrate 15, the substrate 15 is loaded on the transfer plate 31, and the alignment film coated on the substrate 15 is rubbed in a selected direction using the roller 30 wound with cloth such as velvet to align the alignment film 10.
However, the rubbing method according to the related art requires a direct contact between the alignment film and the roller, causing various problems such as contamination due to generation of particles, a device failure due to generation of static electricity, necessitating a cleaning/drying process after the rubbing process, non-uniformity in large-sized application, or the like.
To address these problems associated with the related art rubbing method, a variety of alignment methods have been suggested, such as a method using an LB film, a method using UV illumination, a method using oblique evaporation of SiO2, a method using micro-groove, and a method using an ion beam.
The alignment method using an ion beam can use a conventional alignment material (the same type of material disclosed in the related art) without any change and can be used for a large-sized panel, according to U.S. Pat. No. 5,770,826.
More specifically, the alignment method using an ion beam uses an ion beam having a low energy in which the ion beam bombards the surface of an alignment film, for example, a polyimide film, so that the surface of the alignment film has a directionality. This alignment method is advantageous in that an alignment pattern can be formed without any direct contact with the alignment film at a low energy. In addition, because the ion beam only influences the chemical bonds on a surface of the alignment film, the number of radicals that are formed when chemical bonds are broken can be minimized, and a uniform alignment pattern can be obtained.
The alignment method using an ion beam will now be described in detail. FIG. 3 is a schematic view illustrating ion beam irradiation equipment according to the related art, and FIG. 4 is a graph showing a relationship between an irradiation angle of ion beam and a pretilt angle.
Referring to FIG. 3, an ion beam irradiation part 2 ionizes a gas supplied from a gas supply part 1 into ions and irradiates the ions (ion beam) onto a substrate 4. The ion beam treatment is performed inside a process chamber 8. The process chamber 8 further includes a gas discharge part 9 disposed at a side of the process chamber 8 for discharging gas remaining after the ionization, and a support bar 7 for supporting a substrate 3. To adjust the projection time, the ion beam irradiation equipment may include a shutter 4 between the ion beam irradiation part 2 and the substrate 3.
To form a pretilt angle on an alignment film, an ion beam is irradiated while the substrate 3 is positioned oblique to an irradiation direction of the ion beam at a predetermined angle. Assuming that an angle between an irradiation direction of the ion beam and a direction perpendicular to the substrate 3 is ‘θ’, a relationship between the angle ‘θ’ and the pretilt angle is shown in the graph of FIG. 4. In other words, as shown in FIG. 4, the pretilt angle varies depending on the angle ‘θ’, which indicates that a desired pretilt angle can be obtained by irradiating the ion beam on the substrate at a particular angle.
However, the alignment method using an ion beam according to the related art has the following drawbacks. That is, because the ion beam is irradiated while the substrate is positioned oblique at an angle, as illustrated in FIG. 3, it is required to maintain a sufficient distance between a point where the ion beam starts to propagate and the substrate to irradiate the ion beam of uniform energy onto the substrate. In addition, as the size of the substrate increases, the distance should also increase to maintain the uniformity, thereby increasing the size of the equipment. Thus, as the size of the equipment increases, the fabrication costs increase, the fabrication process becomes complicated, and the production yield decreases.